FILMS AT OIL-WATER INTERFACES. II

Eric Hutchinson 1 From the Department of Chemistry, Stanford University, Palo Alto, Calif.

Received March 12, 1948

INTRODUCTION

In a previous communication (8), data were presented for the inter- facial tensions against water of a series of solutions of fatty acids and alcohols. This work has been extended to higher homologues of the acids and alcohols, and variations in temperature and the nature of the solvent have been studied.

EXPERIMENTAL

In the previous paper the interfacial tension data were obtained by means of the sessile bubble method (4) but it was found that the drop- volume method (6) yielded results in good agreement with these (~6.2 dynes/cm.) provided the systems which were studied showed no marked aging. One difficulty with the sessile bubble method, however, lay in the poor thermal control which was achieved. Hence, in the present work the results have, in general, been obtained by the drop-volume method be- cause of the great ease of thermostating; the results being checked where possible, viz., at 25”C., against the sessible bubble method. The agreement was good, and confidence is felt in those results obtained at 1O’C. and 40°C. which could not be checked against the sessible bubble method. Previous work indicated that, with solutions of the lower members of the fatty acids and alcohols, the ring method (7) occasionally gave er- roneous results due to variations in the contact angle on the ring. For higher members, notably cetyl alcohol and myristic acid, the ring method gave results in good agreement wit,h the drop-volume and sessile bubble methods.

As the details of the methods of measuring interfacial tension were given in the previous paper, no further description will be given here.

Freezing point data necessary to the ,calculation of the p’s were ob- t,ained, as before, by a standard Beckmann method using 25 ml. of solution.

The caprylic and myristic acids, cetyl alcohol, benzene, cyclohexane, tend nitrobenzene were supplied by Eastman Kodak Company. The n-octyl alcohol and n-tetradecane were supplied by the Connecticut Hard

1 Bristol-Meyers Compan,y Postdoct,orate Fellow :tnd Rwertrch Associate in Chcm- isir,y.

23.5

236 ERIC HUTCHINSON

Rubber Company. The myristic acid required 3 recrystallizations from acetone solutions before “aging,” due to impurities, became negligible.

RESULTS

Values for the interfacial tensions against water for n-octyl alcohol and n-caprylic acid in benzene at 10°C. and 40°C. are given in Tables I-IV, values for n-octyl alcohol and n-caprylic acid in a number of solvents in Tables V-IX, and values for cetyl alcohol in benzene, and myristic acid in benzene and cyclohexane in Tables X-XII.

Calculations of the surface excesses r require a knowledge of the freez- ing point depression @ of the solution under investigation. In the case of cetyl alcohol and myristic acid the low solubilities at ~5%. preclude the possibility of measuring 0 for all the solutions which were used. Since such data for dilute solutions as could be obtained fell on the general 0 - N, curves for homologues given in the last paper, the assumption has been

TABLE I

Caprylic Acid in Benzene at 10°C’.

0.358 0.218 0.122 0.0584 0.0285 0.0094 0.000

N.

0.358 0.218 0.122 0.0893 0.0584 0.0285 0.0094 0.0047 0

I I

_-

-

--

l?Ob dynes/cm

13.6 7.99 16.3 4.71 18.6 2.040 22.1 1.002 25.1 0.353 27.8 0 35.0

0.588 0.279 0.139 0.0621 0.0293 0.00948

-

_-

-

e

lZ5 7.99 4.71 3.23 2.04 1.002 0.353 0.180 0

0.210 0.445 0.750 1.75 3.60

10.0

O . - V WL wjcm2. > o-10

3.14 1.97 4.74 3.16 2.47 4.13 2.65 2.31 4.09 2.76 2.60 2.80 2.65 2.57 2.63 2.41 2.39 2.47

TABLE II

Caprylic Acid in Benzene at dOO”C.

iv. Y ii5

______

Y7LKC/CV7%.

12.9 0.588 15.3 0.279 17.6 0.139 18.7 0.098 20.7 0.0621 23.1 0.0291 26.7 0.00948 27.7 0.00473 33.8

ar ae

0.210 0.450 0.752 1.10 1.74 3.55

10.3 19.7

r(l) I I r(N) r(u)

g.-moles/cm~. > 3.14 1.98 3.18 2.50 2.66 2.33 2.74 2.50 2.74 2.59 2.63 2.55 2.44 2.39 2.37 2.35

lo-‘0 4.74 4.18 4.09 3.13 2.80 2.63 2.51 2.40

~- D

3;:s 40.0 50.9 58.8 62.7 66.9

-___

F

lynes /cm. 21.4 is.7 16.4 12.9

9.9 5.2

A F

-___

3:8 dynes/cm. 20.9 39.5 18.5 50.9 16.2 52.7 15.1 58.8 13.1 62.7 10.7 66.3 7.1 68.8 6.1

FILMS. II 237

TABLE III n-O&y1 Alcohol in Benzene at 10°C.

B

3% 2.628 1.946 1.261 1.070 0.825 0.597 0.350 0

= X X

5 4

=

_-

=

-

ZZ

. -

N, J-f ae

N. .- N.

0.140 0.0995 0.0625 0.0290 0.0235 0.0165 0.0097 0.0054 0

A F

______ D

2::3 dynes/cm. 20.6 29.5 18.9 35.0 17.7 48.8 15.1 51.3 14.2 56.4 12.7 69.1 10.5 73.6 6.8

Iynes fcm

14.4 16.1 17.3 19.9 20.8 22.3 24.5 28.2 35.0

1.73 2.16 2.92 4.50 5.30 6.90 9.52

16.25

8.4 s/cmz. % 0 -10

6.05 5.30 6.29 5.37 4.88 5.59 4.56 4.29 4.71 3.26 3.17 3.39 3.11 3.04 3.22 2.84 2.80 2.92 2.32 2.30 2.39 2.21 2.20 2.25

0.123 0.0904 0.0589 0.0282 0.0230 0.0162 0.00968 0.00541 0

-

TABLE IV n-Octyl Alcohol in Benzene at 40°C.

E.Z = ZZZ

r(l) I I .+C -f(u) A F N. xc !!!I

JO -__

0.140 1.73 0.0995 2.16 0.0625 2.92 0.0290 4.50 0.0235 5.30 0.0165 6.90 0.00975 9.52 0.00544 16.25

8

--

v. 3.281 2.628 1.946 1.261 1.070 0.825 0.597 0.350 0

Y __- /nes/cm 16.2 17.2 18.7 22.3 22.8 24.8 25.8 29.1 33.8

-.- dz

26.3 29.5 35.0 48.8 51.3 56.4 69.1 73.6

Jv,

-- Y7lG/C7fC.

16.6 15.8 14.1 10.5 10.0 8.0 7.0 3.7

I I _- .- g&n

6.05 5.37 4.56 3.26 3.11 2.84 2.32 2.21

s/cm2. : 5.30 4.88 4.29 3.17 3.04 2.80 2.30 2.20

0-m

6.29 5.59 4.71 3.39 3.22 2.92 2.39 2.25

cl

Z.ZZ

--

d.

0.123 0.0904 0.0589 0.0282 0.0230 0.0162 0.00968 0.00541 0

-

TBBLE V n-Octanol in Cyclohexane at ZS”C.

0

---~ ‘T.

0 7.35 4.62 2.77 1.484 0.949 0.693 0.494 0.363 0.190 0.100

Y

,nes/crr 49.6 16.1 18.1 20.0 23.4 26.6 31.0 34.3 37.0 41.3 44.2

-

~- P

a.-m s/cm~. > yaeslcm.

0.172 0.4 7.0 5.97 7.38 22.4 33.5 0.0762 1.0 7.75 7.21 7.82 21.1 31.5 0.0361 1.7 6.24 6.03 6.32 26.1 29.6 0.0127 4.7 6.07 6.01 6.14 26.9 26.2 0.00660 8.8 5.91 5.90 6.01 27.5 23.0 0.00345 16.6 5.65 5.65 5.72 28.9 18.6 0.00226 24.5 5.63 5.63 5.63 29.3 15.3 0.00158 31.2 5.02 5.02 5.02 32.9 12.6 0.00082 31.2 2.60 2.60 2.60 63.4 8.3 0.00042 31.2 1.33 1.33, 1.33 124.0 5.4

-

0 0.147 0.0708 0.0348 0.0125 0.0065 0.0034 0.0022 0.0015 0.0008 0.0004

--

5 2 '4 6 ,16 23

238 ERIC HUTCHINSON

TABLE VI

n-Caprylic in Cyclohexane at %YC.

w.

0 0.0785 0.0486 0.0330 0.0168 0.00852 0.00426 0.00155 0.00040

iv, --

0 0.0159 0.0314 0.0609 0.0888 0.115 0.139 0.245

--

Q N.

Y -- N”

oc. yneslcm. 0 48.2 9.45 20.7 0.0852 5.85 22.9 0.0511 4.01 24.9 0.0342 2.126 27.8 0.0171 1.070 30.9 0.0086( 0.525 33.6 0.0043 0.194 37.2 O.b015( 0.040 41.1 0.0004(

JY iii?.

--

=

.- r(l) I I $N) r(u) A

g.-moles/cm~. fo-‘fi

0.487 4.12 3.79 4.43 37.3 0.788 3.99 3.80 4.22 39.1 1.145 3.88 3.75 4.00 41.2 2.17 3.68 3.61 3.77 43.8 4.30 3.66 3.64 3.71 44.4 8.77 3.74 3.73 3.69 44.7

15.9 2.46 2.46 2.46 67.2 58.2 2.31 2.31 2.31 71.5

-

z

--

4

-

=

_- F

ynes/cm.

27.5 25.3 23.3 20.4 17.3 14.6 11.0

7.1

TABLE VII

n-Octyl Alcohol in Nitrobenzene at 15’C.

N. 5%

0.0162 0.0324 0.0648 0.0975 0.130 0.162 0.324

$1) J+) Q.-moles/cm=. x 10-a

I I

F e Y ~-

OC. &WS/~ 0 24.7 0.795 21.0 1.415 18.8 2.254 16.0 2.792 14.8 3.168 13.6 3.440 13.1 4.450 11.1

4.10 3.60 2.90 2.50 2.20 1.90 1.50

1.50 2.65 4.27 5.54 6.50 6.97

12.6

1.48 1.58 104.0 2.57 2.76 59.8 4.00 4.46 37.1 5.04 5.70 29.0 5.75 6.64 24.8 6.00 7.01 23.6

Y7L~S/lWL.

3.7 5.9 8.7 9.9

11.1 11.6

TABLE VIII

n-Caprylic Acid in Nitrobenzene at B5"C.

N, - No F

--

ynes/cm.

0.0157 4.75 1.67 1.64 1.76 97.7 4.3 0.0322 2.95 2.13 2.06 2.28 72.2 5.8 0.0644 1.82 2.62 2.46 2.89 57.2 7.8 0.0941 1.47 3.09 2.83 3.44 48.0 9.5 0.126 1.23 3.46 3.08 3.89 42.5 9.8. 0.160 0.98 3.50 3.02 4.06 40.6 10.5 0.323 0.57 4.11 3.11 4.86 34.0 12.4

Y

‘ynesfcn E.

24.9 20.6 10.7 17.1 15.4 15.1 14.4 12.5

- -

N. Q

-- OC.

0 0 0.0154 0.56 0.0312 0.951 0.0605 1.746 0.0860 2.432 0.114 3.079 0.138 3.73 0.244 6.42

-

N.

0.290 0.154 0.0793 0.0291 0.0133 0.00665 0.00332 0

h-. --

0.0595 0.0313 0.0214 0.0144 0.00772 0.00404 0.00148 0

zz=

--

-

=

_-

-

=

_-

-

--

N.

0.0435 0.0292 0.0197 0.00994 0.00664 0.00363 0.00182 0

fl

2% 1.310 0.670 0.245 0.115 0.060 0.030 0

0

1~~~ 1.308 1.01 0.750 0.470 0.300 0.100 0

0

$4 0.78 0.54 0.305 0.215 0.100 0.050

= I d

-

FILMS. II

TABLE IX

Caprylic Acid in I’etmdecane at Z5’C.

239

2.30 4.50 7.60

18.1 29.6 32.4 68.4

g.-noZes/cm~. > 4.40 3.11 3.83 3.23 3.06 2.82 2.54 2.46 1.86 1.84 1.01 1.00 1.08 1.07

<1

- L

TABLE X

Y

ynes/cm 18.3 21.4 23.8 25.6 28.2 30.8 3x3 34.9

Cetyl Alcohol in Benzene at .25T.

N. x

ar as

____

0.0633 3.82 0.0323 5.78 0.0219 7.50 0.0146 8.4 0.00775 10.4 0.00407 13.2 0.60149 16.8

‘O-10

5.39 4.46 3.43 2.68 1.95 1.01 1.10

=

g.-moles~cm~. x 10-0 6.36 5.98 6.54 4.91 4.76 5.04 4.32 4.23 4.44 3.22 3.18 3.30 2.12 2.11 2.17 1.42 1.42 1.44 0.654 0.654 0.654

=

--

(

-

-

A F

-___ D

3::6 &mes/cm. 33.2 37.1 29.5 48.1 26.0 61.6 21.2 84.9 18.1

165) 15.0 150 12.2

A F

______ 0

2z3 dynes jcm.

16.3 32.7 13.4 37.2 11.1 50.0 9.3 76.1 6.7

115 4.1 252 1.6

TABLE XI

Myristic Acid in Benzene at 25°C. =

I- ~(1) / 19’) / r(U) j A F

gma/cm 23.4 24.8 26.7 28.3 29.5 30.8 31.8 35.1

0.0455 3.20 0.0301 4.68 0.0201 6.76 0.0101 10.3 0.0067 14.05 0.00363 18.1 0.00182 25.0

-

g.-moles/cm? o-10

3.83 3.65 3.98 41::4 3.70 3.60 3.83 43.1 3.57 3.50 3.59 44.9 2.74 2.71 2.80 59.0 2.49 2.48 2.49 66.7 1.72 1.71 1.75 94.0 1.19 1.18 1.20 138

4

-

ynes/cm . 11.7 10.3

8.4 6.8 5.6 4.3 3.3

240 ERIC HUTCHINSON

TABLE XII

Hyrislic Acid in Cyclohemne at 26°C.

N,

0.0366 0.0270 0.0179 0.00862 0.0058 0.00316 0.00085 0

ZZ

-

9

22 3.27 2.77 1.07 0.73 0.39 0.10

=

_- --- L!?l43slcm.

28.7 0.0380 (0.6) 29.5 0.0277 1.2 31.0 0.0181 1.85 33.8 0.00867 3.80 35.4 0.00582 5.20 37.0 0.00317 7.00 40.7 0.00085 21.65 49.8

-

f(l)

I I

+w r (u)

g.-md (2.26)

3.29 3.24 3.26 3.00 2.20 1.83

eslcvo. >

3.21 3.19 3.24 2.99 2.19 1.82

A F

-- Aa dynes/cm.

3.49 47.3 20.3 3.35 49.3 18.8 3.32 49.8 16.0 3.06 53.9 14.8 2.23 74.0 12.8 1.83 90.2 9.1

=

made that 8 for more concentrated solutions can be read from the general curves.

In Figs. l-10 are plotted interfacial tensions against freezing point depression 0 and mole fraction of surface active material N,. Due to the close linearity between 0 and N, in. the case of the fatty acids the y - 6 and y - N, curves are practically indistinguishable but this is not so for the alcohols.

I I I oa5 I.0 I.5 i

MOLE FRPCTION OF ALCOHOL No

N CCTYL ALCOHOL IN BENZENE

w f-0 10°C t-o g-e 4dC o--o I-6 10°C O--O I-NS40“C

I I I I 1 FREEZIN; POlNT DEPRE&ON e

3

FIG. 1. Interfacial tension VS. freezing point depression for solutions n-octyl alcohol in benzene.

of

FILMS. II 241

I I I 0.1 02 03

I hM)LE FRiCTlCN OF ACID ti,-

CAPRYLIC ACID IN BENZENE

0 I O’C

l 40%

IO I I I

5 IO I5 FREEZING POINT DEPRESSION 6

FIG. 2. Interfacial tension vs. freezing point depression for solutions of n-caprylic acid in benzene.

5

O-

5-

o-

I I I 005 0.1 0 0 I5

MOLE FRACTION OF ACID N,

I I I I I 2 3 4

FREEZING POINT DEPRESSION @

FIG. 3. Interfacial tension vs. freezing point depression for solutions of n-caprylic acid in nitrobeneene.

242 ERIC HUTCHINSON

I I I I I I

50 2 4 6

MOLE FRACTION OF ACID

N CAi??YLlC ACID IN N CAi??YLlC ACID IN

40 - 40 - C~C?cft~EXnhE C~C?cft~EXnhE 5 5

2 2 : : 0 0

&\ &\

230 230

$20 - $20 - t t w w t- t- I I

I I I I I I

FREEZ:NC POINT &PRESSIO: 8 FREEZ:NC POINT &PRESSIO: 8

FIG. 4. Interfacial tension vs. freezing point depression for solutions of wcaprylic acid in cyclohexane.

b 605 tie oi5

50 MOLE FRACTION OF ACID NS

I FREEZING POINT DEPRESSION @

FIG. 5. Interfacial tension vs. freezing point depression for solutions of n-caprylic in n-tetradecane.

FIG.

FIQ.

FILMS. II

I I I 005 0.10 0.15

50 MOLE FRACTION OF ALCOHOL N,

40 N OCTYL ALCOHOL IN

CYCLOHEXANE

3

t I I 1 I I

2 4 6 FREEXNG POINT DEPRESSION @

6. Interfacial tension vs. freezing point depression for solutions of +octyl akohol in cyclohexane.

I I I 010 0.20 0.30

MOLE FRACTION OF ALCOHOL N.

N OCTYL ALCOHOL IN

NITROFJENZENE

I I I I I 2

FREEZING POINT DEPR&ON 19 4

7. Interfacial tension vs. freezing point depression for solutions of n-octyl alcohol in nit,robenzene.

243

244 ERIC HUTCHINSON

I I I I 40

t

ob2 ok4 0.06 MOLE FRACTION OF ALCOHOL Nr

CETYL ALCOHOL IN

BENZENE

I I

FREEZING PRINT DEPRESSION ‘6

FIG. 8. Interfacial tension vs. freezing point depression for solutions of cetyk alcohol in benzene.

oA2 t MOLE FRACTION OF ACID N<

MYRISTIC ACID IN

I I I FREEZING ‘$,NT DEPRESSIO~‘“~

FIG. 9. Interfacial tension vs. freezing point depression for solutions of myristic acid in benzene.

FILMS. II 245

10

I

O-

O-

O-

I I I I I I I 0-01 002 003

r MOLE FRACTION OF ACID N,

MYRISTIC ACID LN

I 3 FREEZING PO:NT DEPRESSION 0

4

FIG. 10. Interfacial tension vs. freezing point depression for solutions of myristic acid in cyclohexane.

Finally in Figs. 11-13 are drawn force-area curves for the adsorbed films, the area per molecule being calculated as shown below. the area per molecule being calculated as shown below.

I I I I I I 20 20 40 40 60 60

[iN‘STRd; [iN‘STRd; 100 100

AREA PER MOLECULE AREA PER MOLECULE

FIG. 11. Force-area curves for n-octyl alcohol in various solvents.

246 ERIC HUTCHINSON

Fro. 12. Force-area Curves for n-caprylic acid iu various solvents.

20 40 AREA PER MOCEC”C~” [bmd~;;

Pm. 13. Force-area curves for myristic acid in benzene and cyclohexeue.

The calculation of the surface excess of the film-forming compound, I”” pv, r(u) as defined by Guggenheim and Adam (5), follows exactl> the’treatkent’given in the previous paper, viz.

FILMS. II

and

247

AJ-') + N, '(“) = A,N, + A$,' (3)

where A, = cross-sectional area of solute molecule,

A, = cross-sectional area of solvent molecule.

The area, A, per solute molecule in the adsorbed film is given by A = l/F”) and the force F = y - y0 where y0 is the interfacial tension of the pure solvent against water.

It was suggested in the previous paper that oil-soluble compounds adsorbed at the oil-water interface are oriented with the polar group in t,he interface and the hydrocarbon chain entirely in the oil phase. This conclusion was based on consideration of the energetics and on the .re- markably small effect of increased chain length compared to correspond- ing effects at air-water surfaces. The force-area curves for n-caprylic acid and myristic acid, and for n-octyl alcohol and cetyl alcohol, show that doubling the chain length has very little effect on the properties of the film

Adam (1) makes the general statement that, at oil-water interfaces, the lateral adhesion between the hydrocarbon chains iaso reduced that a material which forms a “liquid-expanded” film on water forms a “gaseous” or highly expanded film at an oil-water interface. This does not appear to be borne out by the present data, for while it is true that, at a given surface pressure, myristic acid occupies a larger area in the film at an oil-water interface than at an air-water surface, yet, nevertheless, the force-area curve contains a transition point characteristic of “liquid- expanded” films. Further, although the area per molecule is about 100% greater under a given pressure, at an oil-water interface, still the “transi- tion pressure” is actually lower by cu. 10 dynes/cm. Similarly, the lower homologues occupy an area in the film at oil-water interfaces approxi- mately twice that at an air-water surface, at a given pressure. This may be described as an “expansion” in going from an air-water to an oil-water interface. On the other hand, however, the fatty acids C4 - Cl4 yield force-area curves with a transition point and a shape characteristic of “liquid-expanded” films on water. This is in sharp contrast to their behavior at- an air-water surface, where the fatty acids C4 - Clz form imperfect “gaseous” films (at cu. 2O’C.) in which lateral adhesion be- tween neighboring film molecules is small. Hence, if the particular shape of force-area curve is truly indicative of a “liquid-expanded” film at an

248 ERIC HUTCHINSON

oil-water interface, then the fatty acids would appear to form films at an oil-water interface in which lateral adhesion is greater than at air- water surfaces. In this event the area per molecule in the film at a given surface pressure would not be the sole criterion for determining whether the film is more “expanded” at an oil-water interface. The whole force- area curve would need to be considered.

There is, moreover, some basis for reasoning that, although the area per molecule in the film may be greater at an oil-water interface, lateral adhesion in the film may be greater.

The films discussed here resemble expanded films, as discussed by Langmuir (9), in that the polar groups are relatively far apart except under high compression. However, Langmuir considers that, at an air- water surface, the hydrocarbon chains form what might loosely be termed a liquid, in which the forces operating between the chains are the co- hesive forces of pure paraffin liquids. Increased temperature causes the film molecules to “evaporate” from an environment which is essentially a liquid to one in which the molecules are relatively far apart and the forces between neighboring molecules are small. Thus, the latent heat of “evaporation” is considerable, cu. 2 kcals./mole, and the effect of tem- perature on the film properties is quite marked.

Now, in the suggested model for a film at an oil-water interface, solvent molecules are present in addition to the film molecules and the closeness of approach between hydrocarbon chains will be governed by the balance of cohesive forces between the chains themselves, and between the chains and the solvent molecules. A separation of hydrocarbon chains from a state in which they are close neighbors to one in which they are rela- tively far apart would merely remove the film molecule from an environ- ment of its fellows to one in which solvent molecules are still close neighbors. Thus, the change in force field, and hence the latent heat, corresponding to such a separation or “expansion” would be much less than for the similar change at an air-water surface.

Experiments using n-octyl alcohol and n-caprylic acid at 10°C. and 40°C. show clearly the small effect of temperature changes, in accordance with this hypothesis.

Such effects of temperature, moreover, would be greatest for systems in which the cohesion between the hydrocarbon chains and the solvent molecules is small, so that under high compression the film molecules can squeeze out solvent molecules and approach one another closely.

In fact, however, general observations on the relative solubility of paraffin derivatives in paraffins and other solvents suggest that inter- actions between hydrocarbon chains and many solvent molecules, e.g., benzene, are considerably strohger than between the chains themselves. Thus, there is a strong likelihood that, even at high surface pressures, the

FILMS. II 249

film molecule will be preferentially surrounded by solvent molecules rather than by its fellows.

Variations in the cohesive forces between solvent molecules and hydrocarbon chains should be reflected in the properties of the films adsorbed from various solvents. The greater the interaction the less will the film approximate to the “gaseous” type encountered at air-water surfaces. Thus, due to this lateral adhesion, we should expect films ad- sorbed from benzene solutions to be less “gaseous” or more “condensed” than those adsorbed from paraffin solutions. Experiment shows that this is indeed so ; in the case of tetradecane the film molecules of n-caprylic occupy a much greater area, at a given surface pressure, than they do when benzene is the solvent. Cyclohexane solutions behave in much the same way as benzene solutions, as might be expected from a comparison of their respective solvent properties as regards fatty acids, alcohols, etc. Films of n-caprylic acid occupy even less area, at a given pressure, when adsorbed from nitrobenzene solutions. At first sight this would appear to be anomalous, in that nitrobenzene exhibits an inferior solvent power compared to benzene for fatty acids. However, it is probable that at the oil-water interface the -NO2 group is oriented toward the water so that polar-head interaction occurs in addition to the interactions between the benzene ring and the hydrocarbon chain.

However, although in general the results are in agreement with the proposed model for the film at an oil-water interface, much work remains to be done before any general quantitative conclusions may be reached. Many of the solvents which would be of interest in this investigation have freezing points inconveniently low for osmotic determinations and re- course to boiling point measurements will be necessary in future work.

ACKNOWLEDGMENT

The author wishes to express his thanks to Professor J. W. McBain, F.R.S., for kind encouragement and advice.

SUMMARY

Experiments on the properties of films adsorbed at oil-water inter- faces have been extended to include variation of the chain length of the adsorbed molecules, temperature, and the nature of the oil. In general, the evidence supports the hypothesis of an interfacial film in which the polar group is anchored at the interface and the hydrocarbon chain is entirely within the oil phase.

Changes in the degree of “condensation” or “expansion” of the films :&orbed from Y number of solvents support the hypothesis that the co- hesion between the solvent molecules and the hydrocarbon chain of the film molecule is an important factor, and reasons are adduced to show

250 ERIC HUTCHINSON

that lateral adhesion in films adsorbed at oil-water interfaces may be greater, rather than less, than at air-water surfaces.

REFERENCES

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